ABI: exclude consumer-generated JSFunctionCall from private imports

[chrisbbreuer]

Problem

The pinned Home and Bun private-JSC inventories currently classify JSFunctionCall from jsc/FFI.zig as a pending zig-js export. That declaration describes the symbol generated into each runtime-compiled FFI module, not a symbol imported from JavaScriptCore or zig-js. The consumer emits the definition, relocates the module, and resolves it with getSymbol("JSFunctionCall").

Counting it as a pending private ABI symbol makes the compatibility gate demand an incorrect duplicate provider and contradicts the Bun inventory boundary, which already excludes generated bindings.

Scope

• Add an explicit consumer-provided/generated classification to the shared private-ABI scanner.
• Classify JSFunctionCall consistently in the Home and Bun pinned inventories.
• Keep it in the declaration inventory for provenance while excluding it from pending zig-js implementation counts.
• Add regression coverage to prevent exports from overriding this classification.
• Update ABI docs, README counts, and parent issue evidence.

Acceptance criteria

• Home and Bun audits report one fewer pending private import without adding a JSFunctionCall export.
• The stored declaration remains revision-pinned and digest-checked.
• Documentation explains why this is consumer-provided rather than implemented by zig-js.
• Focused ABI audits pass.

[chrisbbreuer]

Completed on main in 0664f6da.

Evidence:

• JSFunctionCall remains revision-pinned but is now classified as consumer_provided in both inventories.
• Home: 431 private imports, 80 implemented / 351 pending, plus one consumer-provided declaration.
• Bun core: 421 private imports, 73 implemented / 348 pending, plus one consumer-provided declaration.
• Refresh recomputes classification before export status, preventing an accidental zig-js export from overriding the boundary.
• zig build home-private-abi-audit bun-private-abi-audit passes.
• README and ABI/integration docs now explain the runtime-generated FFI ownership.

Linked from #163, #164, #140, and #134.